Electrodynamic microphone



G. SCHENKEL ETAL 3,141,070

ELECTRODYNAMIC MICROPHONE July 14, 1964 Filed June 7, 1960 2 Sheets-Sheet 1 all INVENTOR GERRIT SCHENKE L JACOB I. OE HAAN AGi T a y 1964 G. SCHENKEL ETAL 3,141,070

ELECTRODYNAMIC MICROPHONE 2 Sheets-Sheet 2 Filed June 7, 1960 INVENTOR GERRIT SCHENKEL JACOB I. DE HAAN AGE United States Patent Ofifice 3,141,070 Patented July 14, 1964 3,141,070 ELECTRODYNAMIC MICRQPHONE Gerrit Schenkel and Jacob Izaak De Haan, Ein'dhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed June 7, 1960, Ser. No. 34,556 Claims priority, application Netherlands June 24, 1959 9 Claims. (Cl. 179-4155) The invention relates to an electrodynarnic microphone having a cardioid directional characteristic and provided with a diaphragm which is accessible to the sound waves to be converted into electrical energy not only at the front surface but also at the rear. The sound waves communicate with the rear surface of the diaphragm through a duct, which may be a central bore in the core of the magnet system, and also through an air chamber of a certain compliance, which is provided with at least one further aperture in the wall of the housing and also, through an acoustic resistance, communicates with the air chamber behind the diaphragm. The duct and the aperture or apertures each open into the outer air through an acoustic resistance of finite value.

In a known embodiment of such a microphone, the central duct at the rear opens into an additional air chamber provided with apertures which, in combination with acoustic resistances covering the apertures, operates to provide a cardioid directional characteristic in the low-frequency range.

The microphone in accordance with the invention is characterized in that the duct opens into the outer air directly, that is 'to say only through the associated resistance, the preferable adjustable ratio between this resistance and the total resistance closing the aperture or apertures being such that in the low-frequency range a desired frequency characteristic is obtained together with a desired directional characteristic. For this purpose it is essential that the said apertures have the nature of a resistance.

By means of the microphone in accordance with the invention it may be ensured that, while retaining its predetermined directional characteristic, a desired frequency characteristic is obtained in the low-frequency range with the omission of the additional air chamber. Therefore, compared with the above-mentioned known embodiments, an acoustically new construction is obtained which provides a saving in material and size. For this purpose a correct choice of the ratio between the said resistances is necessary.

The theory on which the invention is based can be realized to particular advantage in a microphone provided with a frequency characteristic changeover switch, for example, a speech-music switch. In this latter switch it is required that despite adjustments for the diiferent frequency characteristics of speech and music, thedirectional characteristic must be retained. If, for example, such a microphone is used for speech, the low-frequency response must be reduced. This may be accomplished by increas ing the resistance behind the central duct. However, this adversely affects the directional characteristic. According to the invention, this defect can be corrected by a corresponding variation of the total resistance covering the aperture or apertures. The same applies with due alterations when the frequency characteristic change-over switch is used to obtain in one position a frequency characteristic for speech, with a desired low-frequency reduction, and in the other position a frequency characteristic for speech also, in which this characteristic descends in another desired manner. Such a change-over may be used if the microphone is used in enclosures wherein the level of ambient noise is high. In such an environment it is desirable for the characteristic to be reduced to a more than normal extent in the low-frequency range. It will be appreciated that such a switch may have more than two positions if more variations in the frequency characteristic should be desired, for example, if it should include a position for the recording of music.

The invention will now be explained more fully with reference to a few embodiments shown, byway of example, in the figures. In these figures,

FIGURE 1 shows a microphone in accordance with the invention,

FIGURES 2 and 3 are equivalent circuit diagrams, and

FIGURES 4a-8b show microphones provided with frequency characteristic change-over switches.

FIGURE 1 shows an electrodynamic microphone having a magnet system which comprises a core 5 of permanent-magnetic material (for example Ticonal steel) secured to a soft-iron pot 6, and a soft-iron pole plate 7 to complete the magnet circuit. Between the pole plate 7 and the core 5 there is an air gap 8 in which a coil 9 secured to a diaphragm 1 suspended from the pole plate is adapted to move. The diaphragm is awessible to the sound waves to be converted not only at the front but also at the rear. At the rear, access is by means of a channel 2, which here is designed as a central bore of the core 5 of the magnet system. Access to the: rear of the diaphragm is also achieved by means of a number of apertures 4 in the soft-iron pot 6 and through an air chamber 3 and a number of holes 3' in the pole plate 7 of the magnet system, each of which is covered by a piece of gauze. The duct 2 and the apertures 4 are also covered with gauze strips 2 and 4' respectively, which similarly to the gauze strips covering the holes 3 serve as acoustic resistances.

FIGURE 2 is a circuit diagram of the electrical equivalent of the microphone of FIGURE 1. In this diagram, m and 0 represent the mass and the compliance respectively of the suspension of the diaphragm, m the mass of the air in the duct 2, r the acoustic resistance in which the duct 2 terminates, r the acoustic resistance constituted by the gauze strips covering the holes 3' in the pole plate 7, 0 the acoustic compliance of the space enclosed by the pole plate 7, the pot 6 and the magnet core 5, and finally r, the total acoustic resistance by which the apertures 4 are closed.

The volume velocity of the diaphragm resulting from the sound waves is:

in which p is the sound pressure at the front of the diaphragm, p the sound pressure at the resistance r 1);, the sound pressure at the resistance 1- and Z1 the acoustic impedance of the diaphragm If the apertures 4 are provided at the point shown in 3 FIGURE 1, (p p is very small, so that the term (p p ).1/]'wC I is negligible. Hence:

1 {jab ifl 2 1+ (2%?2) agpal i 1 r 3 i 3+z1 law '4) jwc r +Z1 r +jwm (4) represents the frequency characteristic.

In the low frequencies, the directional characteristic is determined by jalc 1 3 4 The Expressions 4 and 5 show that a desired directional characteristic together with a desired frequency characteristic can be obtained in the low-frequency range with the aid of r and r If the apertures 4 are provided at points other than those shown in FIGURE 1, for example, at the rear of the enclosure of the space 3, 10 is approximately equal to 11 so that the term (Pr-P3) j e -1 from Equation 1 is no longer negligible, and hence the degree of the cardioid effect would be disturbed unless the resistance 4 accordingly is not given too small a finite value. In view of the effect to be obtained by the invention, the ratio r /r must then be chosen accordingly.

In manufacturing microphones it is usual to start from standard components which are used for different types. These components may not simply be suitable for certain applications. For example, the air chamber 3 may have an excessive compliance (0 for the application as a cardioid microphone. This gives rise to the difliculty that either the directional characteristic or the frequency characteristic does not satisfy the requirements. According to the invention, this disadvantage can be obviated by making the resistances 1' and r adjustable, for example, by using gauze strips of greater or lesser density, or by entirely or partly covering one or a number or all of the apertures. Thus, for example, the cardioid characteristic may be matched with the excessive compliance of the air chamber 3 by varying the resistance r closing the apertures 4, after which the frequency characteristic, which is influenced by the variation of r can be corrected by means of r In general, this necessitates another adjustment of r.;. However, it will be appreciated that after a number of adjustments of both resistances the end in view can be reached.

In the above-mentioned design of the microphone provided with a frequency characteristic change-over switch, the two resistances are preferably simultaneously adjustable to the values required for speech and music, respectively. Embodiments of these arrangements are shown in FIGURES 4 t0 8.

In FIGURE 4, the microphone is provided with a switch 10 comprising a cam 11, which can be operated from without and acts upon a U-shaped leaf spring 12 provided at its ends with a resilient material, for example, foamed rubber 13. A strip of gauze 14 is secured between these ends. FIGURES 4a and 4b are a rear view and a plan view respectively of this switch. In the position shown, the spring 12 is substantially expanded, two holes 4 covered by the resistance 4' being closed by the foamed rubber plates 13 while the strip of gauze 14 leaves uncovered the duct 2 covered by the resistance 2. In addition, an aperture covered with a gauze 4" may be provided which is not influenced by the switch. In the position shown, the microphone is adapted, for example, to the recording of music, the requirements of a satisfactory directional characteristic being also satisfied.

In FIG. 5, the cam 11 has deformed the spring 12 so that the foamed rubber plates 13 are entirely disengaged from the gauzes 4', while at the same time the gauze 14 is stretched so that the gauze 2 is covered. As a result, the low-frequency response combined with a satisfactory directional characteristic is reduced, for by removing the foamed rubber plates from the gauzes 4', r is decreased so that the condition for a satisfactory directional characteristic is no longer satisfied. This disadvantage is again obviated by increasing the resistance r since the gauze 14 now covers the gauze 2.

In FIGURE 6, the switch comprises an L-shaped strip 15 provided with two holes 16 and 17. The hole 16 is covered by a gauze 18. Strip 15 also includes a piece of foamed rubber 19 which is adapted to close an aperture 21 covered with a gauze 20. This strip 15 can slide between four lugs 22. The extreme positions are marked by projections 23 on the strip 15. In the position shown, the aperture 21 is free while the gauze 18 is moved in front of a central duct 25 which is provided at the rear of the microphone and is covered with a gauze 24. Thus, the microphone is adapted, for example, to speech recording. In the other position of the strip 15, the central duct 25 is in communication with the outer air through the gauze 24 and the hole 17. The aperture 21 is then closed by the foamed rubber cushion 19. Here also provision may be made of an aperture covered with a gauze 21', which aperture is not affected by movement of the strip 15.

The switch of FIGURE 7 comprises a clamping brace 26 provided with two lugs 27 and surrounding a microphone 30 so as to be capable of sliding axially by means of slits 28 and studs 29. The brace 26 is provided with one or more holes over a number of which a gauze 31 is glued, a gauze 32 being stretched between the lugs 27. In one position (FIGURE 7a) the gauzes 31 cover a number of apertures which are provided in the lateral wall of the microphone 30 and are provided with gauzes 33 (see FIGURE 7b), while the gauze 32 is disengaged from a central duct 35 which is also covered with a gauze 34.

FIGURE 7b shows the other position, in which the brace 26 is axially moved through a distance such that the gauze 33 covering the aperture in the lateral wall of the microphone is uncovered, while the gauze 32 now covers the gauze 34 of the central duct 35. From FIG- URES 7a and 7b it will also be seen that provision may be made in the lateral wall of the microphone of an aperture which is provided with a gauze 36 and remains uncovered in both positions of the switch.

In FIGURE 8, provision is made at the rear of the microphone 38 of an aperture which is covered with a gauze 37 and which may be covered by a switch lug 39 rotatable about a spindle 40. Simultaneously with the movement of the arm a gauze 41 arranged thereon is moved so as to cover or uncover a gauze 42 provided on a central duct 43. In this embodiment also, the rear wall of the microphone may be provided with an aperture which is covered by a gauze 44 and which is not affected by the movement of the arm.

What is claimed is:

1. An electrodynamic microphone adapted to convert sound waves occurring in an air medium external thereto, said microphone comprising, in combination, a vibratory system including a diaphragm having first and second sides, said first side being accessible to said external air medium and said sound waves occurring therein, a magnetic core having a duct disposed therein having first and second openings communicating, respectively, with said second surface and directly with said external air medium, magnetic housing means enclosing said core and forming therewith an air chamber having a given compliance and having at least one first aperture and at least one second aperture communicating with said second side and said external air medium, respectively, means for terminating said second openingof said duct and said second aperture in acoustical resistances having a predetermined ratio with respect to each other to provide a predetermined directional characteristic for a given frequency characteristic, said given frequency characteristic including the low frequency range of the acoustical frequency spectrum.

2. An electrodynamic microphone adapted to convert sound waves occurring in an air medium external thereto, said microphone comprising, in combination, a vibratory system including a diaphragm having first and second sides, said first side being accessible to said external air medium and said sound waves occurring therein, duct defining means composed of magnetic material and having first and second openings communicating, respectively, with said second surface and said external air medium, magnetic housing means enclosing said duct defining means and forming an air chamber having a given compliance and having at least one first aperture and at least one second aperture communicating with said second side and said external air medium, respectively, said duct defining means and said magnetic housing forming a substantially closed magnetic path, means to terminate said duct defining means and said second aperture in acoustical resistances having a predetermined ratio with respect to each other to provide a predetermined directional characteristic for a given frequency characteristic, said given frequency characteristic including the low frequency range of the acoustical frequency spectrum, and means to adjust said resistances to maintain said predetermined ratio for different bandwidths of said frequency characteristic.

3. An electrodynamic microphone adapted to convert sound waves occurring in an air medium external thereto, said microphone comprising, in combination, a vibratory system including a diaphragm having first and second sides, said first side being accessible to said external air medium and said sound waves occurring therein, duct defining means having first and second openings communicating, respectively, with said second surface and said external air medium, means defining an air chamber having a given compliance and having at least one first aperture and at least one second aperture communicating with said second side and said external air medium, respectively, means to terminate said duct defining means and said second aperture in acoustical resistances having a predetermined ratio with respect to each other to provide a predetermined directional characteristic for a given frequency characteristic, said given frequency characteristic including the low frequency range of the acoustical fre quency spectrum, means to adjust said resistances to maintain said predetermined ratio for different bandwidths of said frequency characteristic, and frequency characteristic change over switch means having at least first and second positions, said means to adjust being responsive to said switch means to simultaneously adjust said resistances to said predetermined ratio at each of said first and second positions of said switch.

4. An electrodynamic microphone according to claim 3 wherein said air chamber defining means has at least two second apertures, and said switch means further com- 6 prises a substantially U-shaped leaf spring member having first and second ends, and cam operating means adapted to coact with said spring member to position said switch in said first and second positions, and said means to adjust further comprises cushion members of resilient material disposed on said first and second ends and adjacent, respectively, to each of said second apertures and a gauze material strip member disposed on said spring member between said ends and adjacent to said second opening of said duct defining means, each of said cushion members being adapted to cover at least a part of the respective said second aperture associated therewith and said strip member being adapted to open at least part of said second opening to provide said simultaneous adjustment of said resistances in said first switch positiomand each of said cushion members being adapted to open at least a part of the respective said second aperture associated therewith and said strip member being adapted to cover at least part of said second opening to provide said simultaneous adjustment of said resistances in said second switch position.

5. An electrodynamic microphone according to claim 3 wherein said switch means further comprises a substantially L-shaped member having first and second limb elements adapted to be movable into predetermined spatial relationships with said second opening and said second aperture, respectively, at said first and second switch positions, said first limb member having disposed thereon first and second holes, and said means to adjust further comprises a gauze material strip member disposed on said first limb member to cover said first hole and a cushion member of resilient material disposed on said second limb member, said cushion member being adapted to cover at least a part of said second aperture of said air chamber defining means and said second hole being adapted to open at least part of said second opening of said duct defining means to provide said simultaneous adjustment of said resistances in said first switch position, and said cushion member being adapted to open at least a part of said second aperture and said strip member being adapted to cover at least part of said second opening to provide said simultaneous adjustment of said resistances in said second switch position,

6. An electrodynamic microphone according to claim 3 wherein said air chamber defining means comprises at least one wall member disposed about said duct defining means, said switch means further comprising a clamp brace member having a surrounding member surrounding said wall member of said air chamber defining means, said surrounding member having at least one hole disposed thereon, and lug means disposed on said surrounding member, means to slide said surrounding member axially along said wall member into at least two positions corresponding to said first and second switch positions, respectively, and said means to adjust further comprising a gauze material disposed on said surrounding member to cover said hole and a gauze material strip member supported by said lug means, said gauze material being adapted to cover at least part of said second aperture and said strip member being adapted to open at least part of said second opening to provide said simultaneous adjustment of said resistances in said first position of said switch, and said gauze material being adapted to open at least part of said second aperture and said strip member being adapted to cover at least part of said sec-- ond opening to provide said simultaneous adjustment of said resistances in said second position of said switch.

7. An electrodynamic microphone according to claim 3 wherein said second aperture and said second opening are disposed substantially in the same plane, and said switching means further comprises a rotatable flat member substantially in parallel adjacent relationship with said plane, said flat member having at least one hole disposed thereon, means to rotate said member into at least two positions corresponding to said first and second switch positions, respectively, and said means to adjust comprises a gauze material disposed on said member to cover said hole, said gauze material being adapted to cover at least part of said second opening and said flat member being adapted to open at least part of said second aperture 'in said first position of said switch, and said gauze material being adapted to open at least part of said second opening and said flat member being adapted to cover at least part of said second aperture in said second position of said switch.

8. An electrodynamic microphone adapted to convert sound waves occurring in an air medium external thereto, said microphone comprising, in combination, a vibratory system including a diaphragm having first and second sides, said first side being accessible to said external air medium and said sound waves occurring therein, magnetic means comprising associated core, pot, and pole plate means, said core means having disposed therein a duct having first and second openings communicating, respectively, with said second surface and directly with said external air medium, said pot means and said pole plate means being disposed about said core means to provide an air chamber having a given compliance, said pole plate means having at least one first aperture and said pot means 255 having at least one second aperture, said first and second apertures communicating with said second side and said external air medium, respectively, means to terminate said duct defining means and said second aperture in acoustical resistances having a predetermined ratio with respect to each other to provide a predetermined directional char- References Cited in the file of this patent UNITED STATES PATENTS 2,848,561 Gorike Aug. 19, 1958 2,862,069 Marchand et al Nov. 25, 1958 2,865,464 Gorike Dec. 23, 1958 FOREIGN PATENTS 190,987 Austria July 25, 1957 1,014,595 Germany Aug. 29, 1957 

1. AN ELECTRODYNAMIC MICROPHONE ADAPTED TO CONVERT SOUND WAVES OCCURRING IN AN AIR MEDIUM EXTERNAL THERETO, SAID MICROPHONE COMPRISING, IN COMBINATION, A VIBRATORY SYSTEM INCLUDING A DIAPHRAGM HAVING FIRST AND SECOND SIDES, SAID FIRST SIDE BEING ACCESSIBLE TO SAID EXTERNAL AIR MEDIUM AND SAID SOUND WAVES OCCURRING THEREIN, A MAGNETIC CORE HAVING A DUCT DISPOSED THEREIN HAVING FIRST AND SECOND OPENINGS COMMUNICATING, RESPECTIVELY, WITH SAID SECOND SURFACE AND DIRECTLY WITH SAID EXTERNAL AIR MEDIUM, MAGNETIC HOUSING MEANS ENCLOSING SAID CORE AND FORMING THEREWITH AN AIR CHAMBER HAVING A GIVEN COMPLIANCE AND HAVING AT LEST ONE FIRST APERTURE AND AT LEAST AND SAID EXTERNAL AIR MEDIUM, RESPECTIVELY, MEANS FOR TERMINATING SAID SECOND OPENING OF SAID DUCT AND SAID SECOND APERTURE IN ACOUSTICAL RESISTANCES HAVING A PREDETERMINED RATIO WITH RESPECT TO EACH OTHER TO PROVIDE A PREDETERMINED DIRECTIONAL CHARACTERISTIC FOR A GIVEN FREQUENCY CHARACTERISTIC, SAID GIVEN FREQUENCY CHARACTERISTIC INCLUDING THE LOW FREQUENCY RANGE OF THE ACOUSTICAL FREQUENCY SPECTRUM. 